Europe and Iran are to put on a united front in support of the landmark 2015 nuclear deal at talks in Brussels Thursday as Washington mulls reimposing sanctions on Tehran.

The European Union and the foreign ministers of Britain, Germany and France will unite to defend the accord, which curbed Iran's nuclear ambitions in return for the relaxing of punishing sanctions but which US President Donald Trump has repeatedly criticised and threatened to leave.

While EU diplomatic chief Federica Mogherini wants to keep the nuclear issue separate from other contentious issues with Tehran, Iranian Foreign Minister Mohammad Javad Zarif will also face tough questions about recent anti-government protests which left 21 people dead.

Trump, who in October refused to certify Iran was complying with the deal but stopped short of withdrawing from it, is expected to decide on Friday whether to extend waivers on nuclear-related sanctions against Tehran.

The EU and other world powers have repeatedly warned it would be a mistake to abandon the deal, thrashed out with Iran over 12 years by the US, Britain, France, China, Germany and Russia.

British foreign minister Boris Johnson called the deal "a crucial agreement that makes the world safer".

"It is vital that we continue to work with our European partners to preserve the Iran deal, and with it the security and prosperity it is bringing to the people of Iran and the world," he said.

His German counterpart Sigmar Gabriel agreed, saying the deal had "fenced in an acute crisis" and was now "a central part of our security" for Europeans.

- Iran warning -

According to two US sources, Trump had not made a decision by Wednesday, while Johnson told the British parliament on Tuesday that London was urging "our friends in the White House not to throw it away".

Iran, which on Monday warned the world to get ready for Washington abandoning the deal, has said if the US walks away from the agreement it is ready to give an "appropriate and heavy response".

Zarif, who travelled to Moscow on Wednesday to seek Russian support, criticised what he called Washington's "destructive policy".

"The United States must understand the unity of the international community over the nuclear deal and change their position as a result," Zarif said, urging world powers to "resist the hostile actions" of the Trump administration.

The International Atomic Energy Agency (IAEA) has repeatedly said Iran is keeping up its side of the agreement, most recently in November.

After talks in Moscow on Wednesday Zarif tweeted a warning.

"Everyone agrees it is imperative that ALL live up to their obligations under JCPOA. IAEA has verified Iran's full compliance, but continuation will depend on full US compliance," he wrote. JCPOA is the official name for the deal.

- Punishing Tehran -

Mogherini, who played an important role in crafting the nuclear accord, has vowed to preserve the deal and has lobbied US lawmakers in Washington.

US Congress is working on a way to punish Iran for its continuing ballistic missile programme and meddling in Middle East conflicts such as Yemen and Syria.

Johnson said these issues would be "an important part of our conversation" in Brussels on Thursday -- along with the recent unrest in Iran.

"I will be making it clear to Foreign Minister Zarif, on the subject of the recent protests in Iran, that the right to peaceful demonstration within the law is central to any truly thriving society," Johnson said.

The French foreign ministry said the meeting must go beyond the nuclear issue.

"Reports of missile transfers and Iranian assistance to countries and non-state actors in the (Middle East) region are a grave concern," the ministry said in a statement.

"Iran's actions in the region, particularly in Iraq, Syria, Lebanon and Yemen, must be discussed because they cause tensions that cannot be ignored."

The 28-member EU has condemned the "unacceptable loss of human lives" in the protests and stressed that peaceful protest and freedom of expression are "fundamental rights".